Field of the Invention
Clamping devices for machine tools are devices which can be mounted on the table of a machine tool in order to maintain one or several workpieces in a determined position for machining them. When machining several identical workpieces on an automatic or semi-automatic machine tool, the clamping device must maintain in a reproducible way a succession of several identical workpieces in exactly the same position relative to the machining tools. To this end, known clamping devices comprise two jaws which can often be provided with jaw shoes the shape of which is adapted to the pieces to be machined. When the jaws are tightened, these shoes maintain the workpieces in a precisely predetermined and reproducible position. As in ordinary vices, a threaded spindle is used to press a movable jaw against the workpiece, which itself rests against the other, stationary jaw of the device. Such clamping devices are for instance described in the document PCT/US95/11132. The asymmetric way in which their jaws move with respect to the machine tool can be detrimental when a high precision is required, because small defects on the surfaces of the workpiece gripped by the jaws can become superposed to permissible variations in the positioning of the clamping device itself, and thereby double the actual error in position of the workpiece relative to the machine tool. Further, when for instance a cylindrical workpiece is gripped at right angle to its axis on its cylindrical surface, the radial error due to the imprecisions of the machine tool and of the clamping means can add up if the workpiece is not always oriented in the same way during a series of successive clamping operations. The same problems occurs when, due to a previous machining operation, a series of workpieces which can be clamped in two symmetrical orientations have an eccentricity which is still within the allowances but the orientation of which relative to the clamping means cannot be controlled.